This invention relates to a detector for use in so-called "nondispersive infrared gas analyzer" (NDIR), a flow detector element for use in the detector, and a process for producing the element.
A detector for use in infrared gas analyzers is known that comprises two gas compartments that are filled with a gas showing the same absorption characteristics as the gas to be measured and that are arranged in series with or parallel to a load cell, a gas channel via which the two gas compartments communicate with each other, and a flow detector element provided in said gas channel in a position that blocks it. The flow detector element may be of a thermal type as described in Examined Published Japanese Utility Model Publication (kokoku) Nos. 59-26278/(1984) and 59-24993/(1984), as well as Unexamined Published Japanese Utility Model Application (kokai) No. 56-99462/(1981) and Unexamined Published Japanese Patent Application No. 7-140075/(1995).
As shown in FIGS. 1 and 2, the thermal flow detector element consists of two heaters (a) that are made of a metal (e.g. Ni) foil and that run in a serpentine path and plates (b) that are made of an insulating material such as glass and which support the two heaters (a) in a face-to-face relationship. The plates (b) have an opening (c) and the gaps (d) between adjacent branches of each heater (a) that are located within the opening (c) provide gas flow channels.
With a constant voltage being applied to the heaters (a) so that their temperature becomes higher than that of the gas in the gas compartments by a certain value, the two heaters a provide the temperature profile shown by (i) in FIG. 2 if there is no gas flow. However, if the gas flows through the gaps d as indicated by an arrow, the heater (a) in the upstream position is cooled in accordance with the gas flow rate whereas the heater (a) in the downstream position is heated with the heat taken from the upstream heater (a); as the result, the two heaters (a) provide the temperature profile shown by (j) in FIG. 2. This temperature change causes a change in the resistance of the heaters (a), which is measured with a Wheatstone bridge, thereby detecting the gas flow. Note that the detected gas flow corresponds to the quantity of infrared absorption by the gas of interest (hence its concentration) that is passed through the load cell (not shown) in a nondispersive infrared gas analyzer.
However, the above-described conventional detector has several limitations. First, the resistance of the metal of which the heaters are made does not have a very high temperature coefficient. Second, the heaters cannot be supplied with a very high voltage.
Thirdly, the heating temperature cannot be very high and if it approaches 500.degree. C., the gas in the gas compartments will deteriorate or decompose. Because of these difficulties, the signal output is very small and the detection sensitivity is accordingly low and dependent on the ambient temperature.
According to Unexamined Published Japanese Patent Application (kokai) No. 60-173443/(1985), it is proposed that a pressure detector be used in place of the detector element working as a thermal flow meter. A problem with this proposal is that due to the need to detect a small pressure difference, a large enough signal output cannot be produced to achieve high detection sensitivity.
The conventional thermal flow detector element shown in FIGS. 1 and 2 has the following additional problems. Since all gaps (d) between adjacent branches of each heater (a) that are located within the opening (c) serve as gas flow channels, the channel or flow path area is large compared to the area of the detector element and the gas flow rate is so much retarded that the change in the temperature of the heaters (a) due to the gas flow is insufficient to provide high sensitivity.
That part of the heaters a which is located within the opening (c) (i.e., which is not supported with the plates (b)) must have a sufficient strength to retain shape, so a thick enough metal foil has to be used to compose the heaters (a). However, if the thickness of the heaters (a) is increased, the heat capacity increases correspondingly to slow down the response speed.